The present invention relates generally to the art of wireless communication and, more particularly, to a system which utilizes ultrasonic acoustical pressure waves to transmit and receive audio signals across a medium such as gas, liquid, or solid material. The invention further relates to the art of modulation of audio signals to the ultrasonic frequency range, and to the art of demodulation of audio signals from frequency modulated ultrasonic carrier signals. The invention further relates to the art of inaudible communication, whereby the information contained in the signals is secure and undetectable by radio frequency monitoring.
Radio frequency waves , or electromagnetic radiation in the frequency range of approximately 10 kilohertz to 100 gigahertz, has been utilized for wireless communication systems by civilian and military personnel for decades. Numerous applications of radio frequency communication methods include, to name a few, radio broadcasting, air traffic control, and cellular telecommunications. Radio frequency communication is limited, for practical purposes, to operation within mediums such as air and space. Furthermore, radio frequency methods are inappropriate in some circumstances where communication is required, such as within blasting zones where explosives may be susceptible to unplanned detonation due to radio interference. In addition, radio frequency methods are limited in their ability to provide a secure system to ensure confidentiality of information, which is required by many applications for communication.
Sound waves, or acoustical pressure waves, have likewise been successfully employed as a method of wireless ultrasonic communication across various mediums. Ultrasonic communication is most often utilized in underwater applications because the physical properties of solids and liquids tend to allow waves traveling via molecular vibrations to cover relatively long distances, on the order of the kilometer range. It has been similarly employed for communication over structural matter such as beams or pipes. Ultrasonic communication has generally not been utilized in air for long range communication because radio frequency methods are particularly suitable in air for long range communication, offering suitable and efficient means for most applications.
Some applications, however, require security and inaudibility by radio detectors. Examples of these applications include undercover operations where it is necessary not only that the communication be uninterpretable, but also that the communication be undetectable so as not to alert the presence of such communication. Other applications requiring inaudibility include situations where radio frequency methods are inappropriate, such as, for example, in a blasting zone where the presence of radio frequency waves could unexpectedly set off a detonator or in a factory with sensitive electronics or other components sensitive to electromagnetic radiations.
In applications requiring confidentiality and a high degree of security, numerous schemes have been employed to minimize detection and eavesdropping. These schemes often include scrambling a signal prior to broadcasting and then unscrambling the signal after reception, as well as continual switching from frequency to frequency. The main problem with these existing techniques is that the simple detection of any radio frequency transmission whatsoever, even if the transmitted signals are not decoded or interpreted, indicates the presence of existing communication. Thus, such schemes may not provide sufficient security in operations requiring complete inaudibility. It is desirable, therefore, to provide a communication system which is inaudible by radio frequency detectors.
The invention is disclosed herein in the context of utilizing ultrasonic waves for relatively long range, secure, wireless communication through air. However, by way of example, and not limitation, the disclosed invention is useful in a variety of applications including undercover operations, industrial applications, and many commercial uses in various media.
Prior art ultrasonic communication systems involving the conversion of audio signals to ultrasonic acoustical pressure waves encompass a variety of methods and applications. In the context of the present invention, it may be noted that there are no known prior art communication systems which employ ultrasonic acoustical pressure waves for signal transmission through air for relatively long distances.
Prior art ultrasonic communication systems employ a means of carrying a modulated ultrasonic frequency signal from a transmitter to a receiver. One approach has been disclosed for use in electrical power networks, whereby a two-tone control signal frequency modulates an ultrasonic subcarrier which is then used to frequency modulate the broadcast of a local FM station. The frequency modulated ultrasonic signal is demodulated from the FM broadcast program on the receiving end by receiver circuitry. In this particular approach, however, communication is entirely through radio frequency waves and telephone lines, whereby although a signal is used to modulate an ultrasonic subcarrier, the modulated ultrasonic subcarrier is never transformed from radio frequency signals to acoustical pressure waves. The communication thus remains detectable by radio frequency detectors. It is desirable to employ an alternate communication carrier other than radio frequency waves such that the system is not limited to the use of radio facilities or wire lines.
Another prior art approach for transmitting a modulated ultrasonic frequency signal across a medium is through the conversion of the electronic audio signals to acoustical pressure waves. This technique is employed in many communication systems where radio waves cannot travel useful distances due to the attenuation caused by the properties of the carrier medium, as in underwater communication.
Prior art ultrasonic communication systems employ a means of modulating an ultrasonic frequency signal with an audio frequency signal. Methods utilized have included both amplitude modulation and angle modulation, which encompasses both frequency and phase modulation.
The amplitude modulation techniques used in prior art have encountered the inherent limitation that medium disturbance, e.g. air or water currents, causes additional amplitude modulation of a carrier signal. Thus, unwanted signals from medium disturbance become superimposed on the amplitude modulated carrier, which often results in difficulty recovering a clean original audio signal. Furthermore, amplitude modulation, even when superimposed on a carrier of ultrasonic frequency, may still be audible.
Another prior art approach for modulating an ultrasonic frequency signal with an audio signal is through frequency modulation. One prior art technique feeds an audio signal through a modulator to produce a frequency modulated (FM) radio frequency signal at a predetermined intermediate center frequency. The FM radio frequency signal is then fed into one input of a balanced modulator having a second input of fixed frequency from a local oscillator. The balanced modulator produces two outputs including the sum and the difference of the two input signals, whereby proper selection of the fixed intermediate frequency for the first input and the fixed frequency for the second input produces at the difference output the frequency modulated signal in the ultrasonic range. It would be desirable to eliminate the additional intermediate carrier frequency step.
Prior art techniques for demodulating audio signals from frequency modulated ultrasonic carrier signals in ultrasonic communication systems have utilized digital integrated circuit techniques.
The present invention reveals a technique for inaudible, long range communication through air, as well as other media such as water or solid pipes and beams. Prior art techniques have involved limitation to liquid or solid media or to very short ranges in air.
The present invention also reveals a technique for simple, direct modulation of audio signals onto ultrasonic frequency carrier signals for use in ultrasonic communication systems. Prior art techniques have always involved indirect techniques resulting from the high cost of quality components required to build systems with high noise immunity operating at high frequency with a wide bandwidth.
Similarly, the present invention reveals a technique for simple, direct demodulation of audio signals from ultrasonic frequency carrier signals for use in ultrasonic communication systems. Again, prior art techniques have involved more complicated, indirect techniques for demodulation.